Group trends enthalpy

Vertical exceptions within a group also occur. The E.A. for F is less than that for Cl, for example. The smaller-than-expected E.A. for fluorine can be rationalized because of fluorine s extremely small radius. The addition of an electron to its valence shell would therefore increase the magnitude of the electron-electron repulsions. Consequently, the E.A. for fluorine is somewhat less than that for chlorine. Additionally, the bond dissociation enthalpy for Fj (155 kj/mol) is considerably less than that expected based on the other members of its group. For comparison, Clj, Br2, and I2 have bond dissociation enthalpies of 242, 193, and 151 kj/mol. Other anomalies occur for N and O, whose electron affinities are also less than the group trend would have predicted. By analogy with the F-F bond strength, the N-N and 0-0 bonds are likewise weaker than those for P-P or S-S. In fact, both the hydrazine (N-N) and peroxide (0-0) classes of compounds are particularly reactive. Hydrazine, N2H4, was once used as a rocket fuel, and many peroxides are potentially explosive. 

Figure 7.7 Trends in the standard enthalpies of formation AH] for Groups 3 and 13 trihalides as illustrated by data for MF3 and MBrj.

Explain the trend of decreasing lattice enthalpies of the chlorides of the Group 2 metals down the group. 

The solubilities of the ionic halides are determined by a variety of factors, especially the lattice enthalpy and enthalpy of hydration. There is a delicate balance between the two factors, with the lattice enthalpy usually being the determining one. Lattice enthalpies decrease from chloride to iodide, so water molecules can more readily separate the ions in the latter. Less ionic halides, such as the silver halides, generally have a much lower solubility, and the trend in solubility is the reverse of the more ionic halides. For the less ionic halides, the covalent character of the bond allows the ion pairs to persist in water. The ions are not easily hydrated, making them less soluble. The polarizability of the halide ions and the covalency of their bonding increases down the group. 

Reliable information on the thermodynamic stability of group 13/15 adducts is usually obtained by gas phase measurements. However, due to the lability of stibine and bismuthine adducts in the gas phase toward dissociation, temperature-dependent H-NMR studies are also useful for the determination of their dissociation enthalpies in solution [41b], We focussed on analogously substituted adducts t-BusAl—E(f-Pr)3 (E = P 9, As 10, Sb 11, Bi 12) since they have been fully characterized by single crystal X-ray diffraction, allowing comparisons of their thermodynamic stability in solution with structural trends as found in their solid state structures. 

The observed dissociation enthalpies of f-Bu3Al—E(f-Pr)3 adducts (12.2 kcal/mol 9, 9.9 kcal/mol 10, 7.8 kcal/mol 11 and 6.9 kcal/mol 12) steadily decrease with increasing atomic number of the pnictine, as was expected (Fig. 3). Since steric interactions within analogously substituted adducts should become less effective with increasing atomic radius of the central group 15 element, the observed trend obviously results from the decreased Lewis basicity of the heavier pnictines. 

The main purpose of these final comments is to show a few general trends in the thermochemistry of Group 14 organometallic compounds, helped by some (hopefully) reliable values. And one of the trends is revealed by a rather usual plot1,2, in which the mean bond dissociation enthalpies of the species MR4 (i.e. one-fourth of the enthalpy required to break all the M—R bonds) are represented as a function of the enthalpy of formation of M in the gaseous state. As observed in Figure 4, for R = H and Me, D(M—H) and D(M—Me) increase with the enthalpy of formation (or sublimation) of M. It is noted, on the other hand, that the differences D(M—H) — D(M—Me) vary from 47.7 kJmol-1... 

Figure 2.9 Plots with trend lines of the conventional enthalpies of hydration of the Group 1 cations and Group 17 anions against their ionic radii also included are the values of the conventional enthalpies of hydration of the Group 1 calions minus 1110 kj mol 1 and the conventional enthalpies of hydration of the Group 17 anions plus 1110 kj mol-1...

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